The present invention relates to a measuring method for obtaining, for example, the tilting center of a power roller for a toroidal type continuously variable transmission.
FIG. 13 shows a variator that constitutes a principal part of a double-cavity half-toroidal type continuously variable transmission 10. The transmission 10 comprises an input disc 12 and an output disc 13, which constitute a first cavity 11, and an input disc 12 and an output disc 13, which constitute a second cavity 14. A pair of power rollers 15 are provided between the input and output discs 12 and 13 of the first cavity 11. The outer peripheral surface of each power roller 15 is in contact with the respective traction surfaces T of the input and output discs 12 and 13. A pair of power rollers 15 are also provided between the input and output discs 12 and 13 of the second cavity 14.
Each power roller 15 is rotatably mounted on a trunnion 17 by means of a power roller bearing 16. The trunnion 17 is rockable around a trunnion shaft 18. The input discs 12 are rotatable integrally with an input shaft 20. The input shaft 20 is connected to a drive shaft 21 that is rotated by means of a drive source, such as an engine. The paired output discs 13 are connected to each other by means of a connecting member 22. An output gear 23 is provided on the connecting member 22. A loading cam mechanism 25 is located at the back of the input disc 12 of the first cavity 11. The rotation of the drive shaft 21 is transmitted to the input disc 12 by means of the loading cam mechanism 25. The rotation of each input disc 12 is transmitted to its corresponding output disc 13 through the power rollers 15. As the output discs 13 rotate, the output gear 23 rotates.
As shown in FIG. 7, each substantially hemispherical power roller 15 has toroidal surfaces 30 that touch the respective traction surfaces T of the input and output discs 12 and 13 and a reference surface 31 that extends at right angles to a central axis Q of the roller 15. Curvature centers OL and OR of the toroidal surfaces 30, which are situated bisymmetrically with respect to the central axis Q in a cross section of the power roller 15 along the central axis Q, are kept at a distance D from the central axis Q and a distance A from the reference surface 31 each. Each toroidal surface 30 is a convex surface with the curvature radius r. The toroidal surfaces 30 touch the respective traction surfaces T of the input and output discs 12 and 13 in a tiltable manner. Each traction surface T is a concave toroidal surface with a curvature radius Rt. In FIG. 7, OD designates the curvature center of the traction surface T.
If the curvature center OD of the traction surfaces T of the discs 12 and 13 is deviated from the tilting center of each power roller 15, slipping is caused at contact portions between the power roller 15 and the discs 12 and 13. This slipping lowers the torque transmission efficiency, and rolling fatigue that is attributable to heating shortens the life of the variator. If the deviation between the respective positions of the curvature center OD and the tilting center of the power roller 15 is great, a contact ellipse along which the roller 15 and the discs 12 and 13 are in contact is deviated from the boundaries of the effective traction surfaces. In this case, excessive pressure acts on the boundaries between a part of the contact ellipse and the effective traction surfaces, thereby drastically shortening the rolling fatigue life. Thus, the aforesaid positional deviation also exerts a bad influence upon an appropriate pressure at the rolling contact portions between the power roller 15 and the discs 12 and 13.
For these reasons, it is to be desired that the power rollers 15 should be rotated synchronously and that the tilting center of each power roller 15 and the curvature center OD of the traction surfaces of the discs 12 and 13 should be made coincident while this transmission is driven. To attain this, the curvature radius of the toroidal surfaces 30 of the power roller 15 and the tilting center of the roller 15 must be accurately obtained so that the discs 12 and 13 and the roller 15 can be positioned accurately.
Thereupon, the toroidal surfaces 30 of each power roller 15 are measured. A shape measurer of the straight-moving type and a three-dimensional measurer are known measuring devices for the power rollers 15. An alternative measuring device is developed and described in Jpn. Pat. Appln. KOKOKU Publication No. 59-44561. This device comprises a rotatable spindle, a micrometer attached to the spindle, a probe to be in contact with a curved surface of a workpiece, etc. In this measuring device, the probe is kept in contact with the workpiece to measure the distance of separation of the workpiece surface from a predetermined circle or circular arc. This measuring device can obtain the curvature radius r of the toroidal surfaces 30, distance 2D between the curvature centers OL and OR, distance A from the reference surface 31 to the curvature centers OL and OR, deviation of shape from an imaginary toroidal surface obtained by approximation to a representative circle with the curvature radius r, tilting center of each power roller 15, etc.
Since each toroidal surface 30 has a narrow measurable region, it is subjected to representative circle approximation by computation based on measured values, its radius, curved surface shape error (distance of separation from the predetermined circle or circular arc), etc. are obtained, and the tilting center is obtained from those values. If each toroidal surface 30 that has a slight shape error Δr, such as deformation or waviness, which is caused when the power roller 15 is worked, is subjected to representative approximation, as shown in FIG. 8, therefore, there is a deviation or error between the curvature center OL of a normal shape E-F-G and a curvature center OL′ of an actual shape E-F-G′. In consequence, an estimated position of the tilting center of the power roller 15 is inevitably subject to an error after the roller 15 and the discs 12 and 13 are assembled.
According to a known curved surface measuring method described in Jpn. Pat. Appln. KOKAI Publication No. 8-285506, the curvature radius and curvature center of a concave surface are measured by means of a plurality of reference spheres with different diameters and a cramping member. However, this method serves only to measure concave surfaces and cannot be applied to the measurement of convex surfaces such as toroidal surfaces of power rollers.